A bicycle is normally provided with a rear derailleur active on a cog set, which consists of a series of coaxial toothed wheels (sprockets) of different diameters and numbers of teeth coupled with the hub of the rear wheel.
A bicycle can also be provided with a front derailleur associated with a crankset, which consists of a series of toothed wheels (ring gears) of different diameters and numbers of teeth associated with a pin of the bottom bracket set in rotation by a pair of pedals.
In both cases, the derailleur engages a transmission chain extending in a closed loop between the cog set and the crankset, moving it on toothed wheels of different diameter and number of teeth, so as to obtain different gear ratios.
In particular, downward gearshifting is said when the chain passes from a toothed wheel of larger diameter to a toothed wheel of smaller diameter, and upward gearshifting is said when the chain moves from a toothed wheel of smaller diameter to a toothed wheel of larger diameter. It should be noted on this point that with reference to the rear derailleur, downward gearshifting corresponds to the passage to a higher gear ratio and upward gearshifting corresponds to the passage to a lower gear ratio.
The movement in the two directions of the rear derailleur is obtained through an actuation device mounted on the handlebars so as to be easily maneuvered by the cyclist.
More in particular, in a mechanical gearshift, the rear derailleur is moved among the toothed wheels of the cog set, in a first direction by a traction action exerted by an inextensible control cable that is normally sheathed (commonly called Bowden cable), in a second opposite direction by the release of the traction of the cable and by the elastic return action of a spring provided in the derailleur itself.
The movement of the rear derailleur is obtained according to an articulated parallelogram linkage wherein the sides of the parallelogram are articulated two-by-two with respect to each other, along respective substantially parallel rotation axes, through pins. In particular, such an articulated parallelogram is formed from a support body, intended to remain fixed with respect to the frame, a so-called chain guide adapted for moving the chain between different engagement positions on the toothed wheels, and a pair of articulation arms or connecting rods (generally identified as outer connecting rod and inner connecting rod), which connect the chain guide to the support body in a mobile manner.
The traction or release of the control cable determine respective rotations of the connecting rods with the consequent movement of the chain guide that faces the desired toothed wheel for precise gearshifting.
Normally, the direction in which the movement is caused by the release of the traction of the cable and by the return spring is that of downward gearshifting; vice-versa, the traction action of the control cable takes place in the direction of upward gearshifting, wherein the chain moves from a toothed wheel of smaller diameter to a toothed wheel of larger diameter.
In the actuation device, the control cable is actuated in traction or in release through winding and unwinding on a rotor element, commonly called cable-winding bush, the rotation of which is actuated by the cyclist with a suitable control lever, or with two control levers (a first lever for upward gearshifting and a second lever for downward gearshifting).
In any case, the actuation device must foresee that the cable-winding bush is held immobile in rotation in a number of predetermined angular positions, angularly spaced apart by predetermined indexing angles and corresponding to the different positions of the derailleur required by the different ratios, i.e. on the different toothed wheels of the gearshift. This function is obtained with the so-called indexers, many types of which are known in the prior art, variously active between the cable-winding bush and the fixed casing of the device.
One type of widely-used indexer comprises an indexing bush and a so-called ball-holding disc the interaction of which leads to the predetermined angular positions of the cable-winding bush being stably reached.
In particular, the ball-holding disc and the indexing bush are fitted onto a control shaft set in rotation by the control lever actuated by the cyclist. The ball-holding disc and the cable-winding bush are constrained in rotation to the control shaft, whereas the indexing bush is fixed and does not rotate as a unit with the control shaft.
On the indexing bush there is a recess that engages and acts as a guide for a ball carried by the ball-holding disc. Such a ball is mobile with respect to the ball-holding disc in the radial direction, so as to be able to engage and follow the recess during the relative rotation between ball-holding disc and indexing bush. The recess is formed from a track that extends substantially circumferentially on the indexing bush equipped with sliding areas in which the ball slides alternated by stop areas in which the ball finds sliding stop positions. The stop areas are angularly spaced apart by angles coinciding with the indexing angles of the cable-winding bush.
In order to carry out upward gearshifting, the cyclist actuates the relative control lever and the control shaft is set in rotation making the ball of the ball-holding disc cross a sliding area of the recess of the indexing bush until it reaches a stop area at which it stops. Such sliding of the ball with respect to the indexing bush corresponds to a rotation of the ball-holding disc and, therefore, of the cable-winding bush. The winding of the control cable on the cable-winding bush determines a rotation of the connecting rods of the rear derailleur that positions the chain exactly at the toothed wheel of immediately greater diameter allowing precise gearshifting.
Similarly, in order to carry out downward gearshifting, the cyclist actuates the relative control lever and the control shaft is set in rotation taking the ball of the ball-holding disc in the stop area immediately before in which it stops. Such a rotation of the ball-holding disc corresponds to a rotation of the cable-winding bush that unwinds the control cable. The release of the control cable determines a rotation of the connecting rods of the rear derailleur that positions the chain exactly at the toothed wheel of immediately smaller diameter allowing precise gearshifting.
Since for the same traction of the control cable the connecting rods of the rear derailleur perform decreasing rotations as the deformation of the articulated quadrilateral increase, in other words increasing with gearshifting operations, and since the cog sets can have the toothed wheels not equally spaced along the axis of the hub of the wheel, the indexing angles of the cable-winding bush, and therefore the angular distances between the stop areas in the indexing bush, are not all the same as each other.
For this reason, the stop areas in the recess of the indexing bush are angularly spaced so that a predetermined angular distance between two stop areas corresponds to a predetermined gearshifting operation, upward or downward, between two predetermined toothed wheels of the cog set.
The Applicant has noted that by increasing the number of toothed wheels of the cog set it is possible to increase the number of gear ratios to the point such as to also be able to hypothesize using a single crankset on the pedal axle, with clear advantages in terms of weight and reliability.
The Applicant has, however, found that the actuation devices of the type summarily described above can be subject to the drawback of limiting the number of toothed wheels able to be used in the cog set.
The Applicant has indeed found that a larger number of toothed wheels in the cog set requires a greater excursion of the rear derailleur that is reflected in a greater stroke of the control cable.
The radial space around the control pin is, however, limited by the bulk of the casing that contains the actuation device (which is mainly dictated by ergonomic reasons), and therefore the diameter of the cable-winding bush cannot be increased as desired to obtain, for the same angular rotation of the cable-winding bush dictated by the rotation of the ball-holding disc, an increase in the stroke of the control cable.
Furthermore, in order to give greater stability and better distribution of the forces inside the actuation device, there are usually two balls of the ball-holding disc and they are diametrically opposite at 180°, each of which engages a respective recess on the indexing bush. In this case, the stop areas are replicated on both of the recesses and are point-for-point diametrically opposite along the circumferential extension of the indexing bush.
The Applicant has noted that in indexers having two balls and two recesses, which ensure high reliability of operation, the useful stroke for the control cable is even limited by a rotation of 180°, or less, of the cable-winding bush.